A method of producing hot-rolled strip having a dual-phase structure composed of fine-grained ferrite (&gt;70%) and martensite grains dispersed therein, wherein the slab essentially containing carbon, manganese, silicon and chromium is first heated to the hot-rolling temperature, then hot-rolled and finish-rolled above A.sub.r3, subsequently cooled rapidly and finally coiled at a low temperature, has been disclosed by European Pat. No. 19,193 and European patent application No. 72,867.
In the method disclosed by European Pat. No. 19,193, the slab containing essentially 0.05-0.20% of C, 0.5-1.5% of Mn and 0.5-2.0% of Si as well as, if appropriate, Cr, V, Mo, Ti and Nb, the remainder being iron, is hot-rolled in the austenitic state, then cooled down to a temperature in the range from about 800.degree. to 650.degree. C., coiled and held for at least one minute at this temperature. Subsequently, the strip is unwound in a further process step, cooled at a rate of &gt;10.degree. C./second to a temperature of &lt;450.degree. C. and finally coiled again at this temperature.
In the method disclosed by European patent application No. 72,867, the slab which has been heated to the rolling temperature and which essentially contains 0.02-0.20% of C, 0.5-2.0% of Mn, 0.05-2.0% of Si and 0.3-1.5% of Cr as well as .ltoreq.0.15% of P and .ltoreq.0.1% of Al, the remainder being iron, is hot-rolled at a final rolling temperature &gt;780.degree. C. After leaving the finishing step, the hot-rolled strip is cooled to an intermediate temperature T.sub.N of the order of about 750.degree.-650.degree. C. at a rate of &gt;40.degree. C./second and held for at least 5 seconds at this temperature. Subsequently, further rapid cooling at a rate of 50.degree. C./second to a temperature in the range from 550.degree. to 200.degree. C. takes place, before the strip is finally coiled at this temperature.
In both the prior methods, a low ratio of the yield point to the tensile strength (&lt;about 0.70) and a good cold formability of the hot-rolled strip, or of the sheet produced from it, are obtained. However, in order to establish the appropriate dual-phase structure, a controlled or stepped cooling process is envisaged in both cases after the hot-rolling or finish-rolling of the hot-rolled strip. Phases of rapid cooling and phases of holding the hot-rolled strip at a defined temperature (cooling in stagnant air) alternate. With respect to equipment, both cooling methods entail expensive cooling sections or, in the case of the prior method of European Pat, No. 19,193, a second uncoiling and coiling device for the finish-rolled hot-rolled strip.
On the other hand, European patent application No. 68,598 has disclosed a method of producing hot-rolled strip having a dual-phase structure, a low ratio of the yield point to the tensile strength and good formability, wherein, as distinct from the two abovementioned methods, the hot-rolled strip, after finish-rolling, is cooled to a low coiling temperature without additional provisions. Essentially, this is achieved by the slab having an increased phosphorus content in the range from 0.04 to 0.20%, in addition to containing 0.03-0.15% of C, 0.6-1.8% of Mn, .ltoreq.0.10% of Al, .ltoreq.0.008% of S and, if appropriate, 0.2-2.0% of Si alone or together with Cr, the remainder being iron. In addition, the slab must be heated to a defined temperature in the predetermined range from 1,100 to 1,250.degree. C., before it is then hot-rolled and, at a temperature in the range from 900.degree. to 780.degree. C., finish-rolled and, after finish-rolling, cooled at a rate in the range from 10.degree. to 200.degree. C./second and finally can be coiled at a temperature of .ltoreq.450.degree. C. Admittedly, this prior method has the advantage that the hot-rolled strip can be produced on conventional mill trains with the associated cooling section without additional expenditure on equipment. On the other hand, however, the increase in the phosphorus content entails a deterioration in the weldability of the hot-rolled strip. Furthermore, the tendency of the hot-rolled strip to temper embrittlement increases with rising or increased phosphorus content. This temper embrittlement especially manifests itself adversely whenever the sheet produced from the hot-rolled strip of increased phosphorus content must then, for example, be welded during further processing. Furthermore, the temperature of the furnace for heating the slab up to the rolling temperature and heating it through and through must be exactly set in this prior method, and in addition this temperature, out of the predetermined temperature range, is below the conventional temperatures.
It is thus the object of the invention to provide a method of producing hot-rolled strip having a dual-phase structure, which strip, coupled with an at least equally good property pattern, namely a low ratio of the yield point to the tensile strength (below about 0.70), isotropic cold workability and good weldability, can be produced by simple means especially on conventional mill trains with an associated cooling section, that is to say without additional expenditure on equipment.